Aruba APs were not setup with static channel and power in mind, so it is not easy to set static AP power. Here is how I would do it:
Setup a new RF dot11a or RF dot11g radio profile that has the channel and power (EIRP) that you want. Setup a new ARM profile that has ARM setting set to disabled and assign it to that radio profile. Lastly, assign that radio profile to an AP specifically:
config t
ap-name <name of your ap>
rf dot11g-radio-profile <name of that radio profile you created>